JOURNAL
OF GEOPHYSICAL
RESEARCH,
VOL. 96, NO. B8, PAGES 13,545-13,560, JULY 30, 1991
Mid-CenozoicStressEvolutionandMagmatismin the SouthernCordillera,Texasand
Mexico:Transitionfrom ContinentalArc to IntraplateExtension
Cm•ISTOPmm
D. Hu•¾, JOS^THA•G. PmCE,
• ANDEmc W. J^•vms
Bureauof EconornicGeology,The Universityof Texasat Austin
Orientations
of dikes,veins,faults,andslickenlines
revealtheevolutionof stressduringEoceneto Miocenemagmatism
in the southern
Cordillera. Wheremostthoroughlystudiedby us in Trans-Pecos
Texas,magmatism
beganat about48
Ma shortlyafter the cessationof Laramidefolding. Dikes and veins that formed from then until about32 Ma strike
dominantly
east-northeast.
This indicates
thattheleastprincipalstress(a•)wasnorth-northwest;
additional
datasuggest
thatthemaximum
principalstress(o0 waseast-northeast.
The stressfieldchanged
to at verticalandaseast-northeast
(i.e., east-northeast
extension)
at leastby 28 Ma andprobablyby 31 Ma. Dikesandveinsthatformedbetween31 and
17Ma, whenall magmatism
ceased
in Texas,strikenorth-northwest.
Thischangemarksthebeginning
of regional,Basin
andRangeextension;
however,majornormalfaulting,exclusively
of high-angle
type, did notbeginuntil about24 Ma.
A similarstress
change,markedby a similarchangein dikeandveinorientations,
occurred
throughout
the southwestern
UnitedStatesandnorthernMexico. The timeof changeis notwell constrained
in Texas,butavailableinformationallows
it to haveoccurredat the sametime thoughout
the southern
Cordillera. We suggest
the earlierstressfield is relatedto
east-northeast
convergence
betweenthe Farallonand North Americanplates. The changein stressis approximately
coincident
with collisionof the East PacificRise andpalcotrench.Extensionmay be relatedto the changefrom a
convergent to a transformmargin along the westernedge of North America. The changesin the stressfield are
accompanied
by changes
in the sources
andcompositions
of magmas
eruptedin Texas. Contemporaneity
of thechanges
in stress
andmagmatism
indicates
thattheyarerelated.Combined
with regionalagepatterns,
palcostress
andgeochemical
dataindicatethatpre-31 Ma magmatismin the southernCordilleraoccurredin a subduction-related,
continentalvolcanic
arc. Subsequent
magmatism
occurredin an environment
of intraplateextension
of the BasinandRangeprovince.
LNTRODUCrlON
to extinguishat about32 Ma, andby 24 Ma, magmatismwas
clearlyrelatedto intraplateextension.This paperandits comCenozoicmagmatismin westernNorth America spanned panion[Jamesand Henry, this issue]usedata on paleostress,
the changefrom a convergentplate margin to the current regional age patterns, and compositionof magmatism to
transformmargin alongmuch of the west coast.Magmatism establishtheoverallevolutionandtectonicsettingof magmatism
duringconvergence
is largelyassigned
to a continental
volcanic in Texas.
arc [Lipmanet al., 1971; Coneyand Reynolds,1977;Damon et
al., 1981]; later magmatismoccurredin a settingof regional
(BasinandRange)extension.
However,thetimingandinfluence
C•ozoIc
M_•o•e•,•sM nq Tms-Pmos
Tvxns
on magmatismandcontinentaltectonicsof the changein plate
marginare complexandpoorlyunderstood,
particularlyin the
southernCordillera [Severinghausand Atwater, 1990]. The Regional Setting
timing and style of regionalextensionhave been especially
Magmatismin Trans-PecosTexasis the easternmost
part of
controversial[Zobacket al., 1981;Lipman, 1981;Elston, 1984; a broad belt of late Mesozoic and Cenozoicactivity in the
Best, 1988; Gans et al., 1989].
southernCordillera, much of which is clearly related to
Texas is a critical area for understandingthe tectonic and subduction(Figures1 and 2). The palcotrenchfor this activity
magmaticevolutionof westernNorth America.Magmatismin lay along the western edge of North America.. At about
Texas representsthe easternmostexpressionof Cordilleran 100 Ma, magmatismwas concentratedalong the Pacific coast.
activity(Figure1) andoccurrednearlycontinuously
from48 to From there,it sweptgenerallyeastward,reachingas much as
17 Ma, spanningthe changein plate margin. The origin of 1000 km inland [Lindgren, 1915; Andersonand Silver, 1974;
magmatismin Texas has itself been controversial,in part Henry, 1975;ConeyandReynolds,1977;Keith, 1978;Damonet
becauseit is not close to any plate margin. Barker [1977] al., 1981]. The pattern of magmatismhas generally been
originallydrew an analogyto the EastAfricanrift on thebasis interpretedto reflect progressiveshallowingwith time of the
of similarity in magma compositions
and in recognitionof subducting
Farallonplate.The Cretaceous
batholithsof western
thepervasivelate Cenozoicnormalfaults.However, faulting Mexico andsouthwestern
United Statesare universallyagreed
postdates
most igneousactivity [Henry and Price, 1986]. We to be related to subduction and to a continental arc [Anderson
now interpretmagmatismup to 32 Ma to have occurredin a and Silver, 1974; Damon et al., 1981; Gastil et al., 1981]. The
continentalvolcanic arc. Magmatism after that time occurred position of magmatismin Texas at the easternend of this
duringregionalextension.The arc eitherextinguishedor began continutunis one part of the evidencethat it alsoconstitutesa
continentalvolcanic arc, despitebeing 1000 km east of the
plate margin.
1Nowat Nevada
Bureau
of MinesandGeology,
University
of Nevada,
After about 30 Ma, magmatismeither swept rapidly back
Reno.
toward the coast or occurred nearly simultaneously(the
ignimbriteflareup) from Texas acrossnorthwesternMexico.
Copyright1991by theAmericanGeophysical
Union.
This marks the beginning of the end of arc activity, and
magmatism
after this time occurredduringregional,intraplate
Papernumber9 IYBO0202.
0148-0227/91/91JB-00202505.00
extension.
13,545
13,546
H•.,•-•m'
sr AL.:Mm-C•ozo•c
S'riu•s EvoLu•o• A•U 1V[AOMATI{M
Colorado
Plateau
Great
Plains
Trans-Pecos
held
Chihuahua
1987]. The data of this reportand of Jamesand Henry [this
issue]indicatethatmagmatismoccurredin two distincttectonic
settings:a continentalvolcanic arc throughabout32 Ma and
regional (Basin and Range) extensionthereafter.All TransPecos magmatism is alkalic, regardlessof age, location, or
tectonicsetting,althoughthedegreeof alkalinityvaries.
The timing of events is critical to the conclusionsof this
report and, except where noted, is well constrained.Geochronological constraintsare provided by approximately
350 K-At ageson more than 200 volcanicandintrusiveunits,
dominantlyon separates
of alkali feldspar,plagioclase,
biotite,
or hornblende[Henry et al., 1986, 1989;Henry and McDowell,
1986].Henry et al. [ 1986] compiledK-At data asof thatdate.
Precisionof theconventionalK-Ar agesfor mid-Tertiaryrocks
is alwaysbetterthan1 m.y. New 4øAr/•Ardataareproviding
muchgreaterprecisionin severalareas[KunkandHenry, 1990].
The isotopicdataaresupportedby andcorrelatedwith detailed
mappingandstratigraphic
studiesthroughout
Trans-Pecos
Texas
[e.g., CepedaandHenry, 1983;Henry et al., 1989].
Magmatismbegan about 4748 Ma with eraplacementof
'•Approximate
limits
of
the
Basin and Range Province
0
2_50 m i
small, mafic to intermediate intrusionsin the northwesternand
ß
southeastern(the Big Bend area) parts of the province
(Figure 3a; [Hoover et al., 1988;Henry and McDowell, 1986]).
Fig. 1. Locationof the Trans-Pecos
volcanicfield, õeneralizeddistribution At the sametime the extensiveAlamo CreekBasalteruptedin
of Tertiary volcanic rocks, and area of late Cenozoicextensionin the the southeast.Magmatism continuedin the southeastbetween
southern Cordillera.
44 and 40 Ma with eraplacementof abundant,small, silicic
to mafic intrusions[Henry et al., 1989] andformationof a small
caldera complex, the earliest in Texas, in the Christmas
Activity in Texas
Mountains[Henry and Price, 1989].
The greatestvolume of magma was eraplacedbetween38
Magmatismin Trans-PecosTexas(Figure3) occurrednearly
continuously
between48 and 17 Ma butvariedconsiderably
in and 32 Ma in two north-northweststrikinggeochemicalbelts:
location,style,composition,
volume,andtectonicsetting[Henry a western,alkali-calcicand an eastern,alkalic belt (Figure 3b)
and Price, 1984; Henry and McDowell, 1986; Price et al., [Barker, 1977; Henry and Price, 1984]. This magmatismwas
t
',
' , ' ,'
0
I
400
Guadalajara
km
QA 5362_
x
20
x
x
Extension 24
30
Continental
volcanic arc
40
.-.. 60
31
38
Field of magmatism
lOO
12o
0
200
400
600
800
1000
Distancefrom trench (km)
QA 15158c
Fig. 2. Timingandlocationof magmatisrn
in Trans-Pecos
Texasin relationto thepaleotrench
thatlay off westcoastof North
America.
Outlined
areaencompasses
agesof magmatism
anddistances
fr•n thepaleotrench
across
northem
Mexicocompiled
by
Damonet al. [1981].Activityin Texasconstitutes
the easternmost
magmatism
in the southern
Cordilleraanddefinestheeastern
limit of themagmatic
sweep.Crosses,
basalts
associated
withBasinandRangefaulting;pluses,
volcanicandintrusive
rocks
emplaced
duringinitialextension
between
31 and27 Ma; circles,volcanic
andintmsiverocksemplaced
duringcontinental
arc
activity.
H,•,m¾
aTAt..:M.m-C•ozoxc
STR•SEVOLtH'ION
AND
•'•iAOMA'I'ISM
48-39
A.
38-32
Ma
106ø
10•
4øNew
Mexico,
•...ii.•
E
"•' X•47-48•
106
ø .
104
øNew
Mexico
I
.;;.36• •
Chihuah•a'X•
'
•e%o•x
'
+31
o
Early
_ -•H
Main
!• \•
oAlpine
•32..•
42
',?
.
106
ø
v.... -•
,
24-17
Ma
L[•
Texas
El Paso
D.
+31 ø
•.
oAlpine
extensional
t,
10•
4øNew
Mexico
j
Cox
Mountain
4•'e+/%
'RimRock Mountains
Davis
+31
o
'•. dikes
•'• i•,24-18
Basin
L
and
Bofecillos
'•ains,•,27-28
•,.,•
,28 /
Texas
Paso
•
+29%
Ma
106ø
•.••'
•
Earl
Alpine
••
+29ø
10,4
øNew
Mexico
I
p o
' • •)36
•.-33XX
,44-4O
• Creek
I;•f• •",A{a•
C.
Texas
I I-'aso
%•,'35-37
• Finlay
Mtns.
31-27
Ma
[•••e, l ...'. x 360.: DiabloPlateau
Texas
I Paso
•
B.
13,547
"•
øAlpine
'•
range•,
Black
GasPs22
'1••'1•82
22 •22
24-2'
24'•+29ø
QAI5157
Fig.3. Distribution
of magmafism
through
timein Trans-Pecos
Texas.
Numbers
indicate
times
ofmajoractivity
ofindividual
centers
orareas
ofmagmafism.
Ruledareas
arecalderas
andsolidareas
aremajor
intrusions.
(a)Initialencroachment
ofvolcanic
are(48-39Ma).Moderate
volumes
ofmagma
wereemplaced
innorthern
andsouthern
Trans-Pecos
Texas.
(b)Maincontinental
arcphase
(38to32Ma).Large
volumes
ofmagma
were
emplaeed,
mostly
fromcaldera
complexes,
throughout
Trans-Pecos.
AC
andA denote
westem
alkalicalcic
andeastern
alkalicbelts.(c)Initialextensional
phase01-27 Ma). Moderate
volumes
of magma
wereemplaeed
exclusively
insouthern
Trans-Pecks
Texas
andChihuahua,
contemporanexms
withinitialeast-northeast
extension.
(d) MainBasinandRange
phase
(24-17Ma).Smallvolumes
of magma
wereemplaced
throughout
Trans-Pecos
Texas
contemporaneous
withmajor
Basin
andRange
faulting.
EP,El Paso.
Letters
nexttocalderas:
X, Christmas
Mountains
caldera
complex;
Q,Quitman
Mountains
caldera;
E,Eagle
Mountains
caldera;
VH,VanHomMountains
caldera;
W,WylieMountains
caldera;
B,Buckhorn
caldera;
EM,E1Muerto
caldera;
P,Paisano
volcano;
IL,Infiemito
caldera;
C,Chinaft
Mountains
caldera;
PC,
PineCanyon
caldera;
S,Sierra
Quemada
caldera;
SC,SanCarlos
caldera;
Sa,Santana
caldera.
to thesouthern
part,in theChinaft
Motredominated
by eruptions
from calderas,occm're.,d
through- partsof theregion
out Trans-PecosTexas, but shifted with time, and was most tainsand Big Bend area.
Magmafism
between
about31 and27 Ma
voluminousin the centraland southernpartsof the region.
wasrestricted
Contemporaneous
noncaldera
magmatism
consisted
of numer- to southernTrans-PecosTexas and adjacent Chihuahua
3c).Caldera-related
volcanism
continued,
astwolarge
oussmallintrusions
andlavasscattered
throughout
theprovince. (Figure
(SanCarlosandSantana
calderas)
formedat 30 and
A northwest
trendingbelt of silica-undersaturated
intrusions calderas
justacross
theRioGrande.
In Texas,the
wasalsoemplaced
at thistime,mostnotablyin theDiablo 28Ma in Chihuahua,
volcaniccentereruptedmaficto intermediate
lava
Plateauof northernTrans-Pecos
(Figure3b) [Barker,1977].At Bofecillos
ofperalkaline
rhyolite
andnepheline-normative
about35 Ma, volcanismshiftedfromthenorthernandcentral flows.Intrusions
13,548
hawaiitewere emplacedin the BofecillosMountainsand Big
Bend area.
After a several million year hiatus, rocks exclusivelyof
basalticcomposition
wereemplacedthroughout
muchof TransPecosTexasasdikes,smallstocks,andsomelavas(Figure3d).
The beginningof this volcanismat 24 Ma coincidedwith the
beginningof significantBasinandRangefaulting.
Volumesof eruptedrocksvariedtremendously
throughtime.
By far the largestvolumeseruptedbetween38 and 32 Ma.
Subordinate,
butstillmajor,volumesof magmaswereemplaced
between48 and 39 Ma and between31 and 28 Ma. Although
widespread,the 24-17 Ma magmatismaccompanyingBasin
andRangefaultingwasvolumetricallyminuscule.
from the centers[Muller and Pollard, 1977]. This pattern
reflectsthe influenceof a pressurizedcircularconduit(the
centralintrusion)on a regionalstressfield [Muller andPollard,
1977].
4. Interpretationof complexdike swarmsrequiresnearly
completeknowledgeof dike distributionandorientation.The
SpanishPeaksdikesshowquiteregularorientations
in each
segmentof arcaroundtheintrusivecenter[Muller andPollard,
1977], but none of theseorientationsreflectsthe regionalstress
field. This problemappliesparticularlyto areasin the Basin
and Range provinceor areasof recurringmagmatismwhere
much of a dike swarm may be buffed beneathbasinfill or
coveredby latervolcanism.
5. Evenif thedikesor veinswereemplacedperpendicular
to
STRESSASnLYS•S n•OM DIKE •
VE• ORIENTATIONS
the minimumprincipalstress,uncertaintyin their age may
This paper usesdike and vein orientationsto infer stress lead to assigment of a stressfield to an incorrectperiod.Age
orientations
duringtheir emplacement4
the paleostress
dataare uncertaintyis particularly a problem in areas of recurrent
wherereheatingor hydrothermal
alterationof dikes
then usedto interpretthe tectonicsettingand evolutionof the magrnatism,
region.A similarapproachhasbeenwidely used[Rehrigand mayresetor altertheirapparentages.
6. Subsequentrotation can producedike swarmswhose
Heidrick, 1976; Price and Henry, 1984; Aldrich et al., 1986;
Best, 1988].More indirectly,Nakamura[ 1977]usedalignment orientation does not correspondto the orientation during
of flank eruptionsarounda volcanoto inferdike orientations emplacement.Vertical axis rotationduringLaramideor Basin
hasbeensuggested
for manyareashi
andpaleostress.
The basicassumption
in all theseapproaches andRangedeformation
southwestern
North
America
[Nelson
and
Jones, 1987;
is that a dike or vein is emplacedperpendicular
to the mini-
mumprincipal
stress
(o3).Thisassumption
onlyholdsif the Hagstrumand Sawyer, 1989].
dike occupiesa mode 1 fracture(a fracturethat opensonly
perpendicular
to its plane [Pollard and Aydin, 1988]) that
formedin response
to the contemporaneous
stressfield, and
the dike hasnot subsequently
beenrotated.A widevarietyof
factorscaninfluencetheproperinterpretation
of stressorientations from dike or vein orientations.
1. A dike or vein may be emplacedinto preexistingfractures
formed in a stressfield unrelated to that during dike em-
For cases1 (preexistingfractures)and3 (local stressfields),
the differencebetween the horizontalstressesis particularly
important.Wherethestressdifferenceis small,dikescanmore
easily intrudepreexistingjoints of any orientation[Delaney
et al., 1986]. Where the stress difference is large, only
preexisting
jointsclosein orientationto theminimumprincipal
stresscan be occupied.Delaney et al. call regionsof small
stressdifferencetectonicallyinactiveandregionsof largedifferencetectonicallyactive.Even in activeregions,horizontal
stresses
generallydiffer by no morethan50% [McGarr, 1982;
Delaneyet al., 1986].
Given theseuncertainties,it is clear that interpretationof
paleostress
from dike and vein systemsmustbe basedon a
thoroughunderstanding
of theirgeology.Both theoreticalconsiderationsand our own data suggestthat hydrothermalveins
within contemporaneous
resurgentintrusionsof calderasare
placement.
Delaneyet al. [ 1986]demonstrated
thatdikescan
occupypreexistingfracturesat variousanglesobliqueto the
stressfield if the magmadrivingpressureexceedsa function
of the two principalhorizontalstresses.
In the extremecase,a
dike can intrudea joint of any orientationif magmapressure
exceedsthemaximumprincipalhorizontalstress.
Additionally,dike orientationeven in a rock withoutpreexistingfractures
maybedetermined
by structures
in underlying
rocks.For example,dikesthatrosethroughjointedbasement the most consistent and reliable indicators of the stress field.
of thepaleostress
field canbe det•mined
rock would,in manycircumstances,
follow thejoints.As the Accurateassessment
from
thoroughly
understood
dike
swarms
evenin olderrocks,
dikespropagated
upwardintounjointed
rocks,theywouldbegin
however.
to reorientto the prevailingstressfield. Whetheror not they
succeeded is a function of several factors including dike
propagationrate, magma viscosity,and distancetraversed
[Emerman and Marrett, 1990]. Although commonly rapid
[Emermanand Marrett, 1990], reorientationmay not occurif
the jointed rocks are shallow. Becausedikes are always
emplacedinto existingrocksandthroughbasement
rocksthat
axecommonlycomplexlydeformed,hostanisotropy
is always
a potentialuncertainty.
2. Dikesor veinsmaybe emplaced
intonon-mode1 fractures.
For example,dikesemplacedinto or alongactivestrike-slip
faultscouldbe substantially
obliqueto theminimumprincipal
stress[Best, 1988].
3. Local stressfields may differ markedlyfrom regional
stressfields.Most critically,dikesemplacedarounda circular
intrusivecentercan be stronglyaffectedby magmapressure
in the center.For example,dikesrelatedto SpanishPeaks,
Colorado,showa radialpatternnearthecentralintrusivebodies
but curve to a more uniform east-northeast
orientationaway
1. Resurgentintrusionsare mostlikely to be isotropic,that
is, to containno fracturesformed in a previousstressfield.
Obviously,this appliesonly to veinsanddikesformedessentially contemporaneously
with the hostintrusion.In contrast,
hydrothermalactivityin areasof locallyrecurringmagmatism,
for example,the San Juanvolcanicfield of Colorado,is in
manycasesmuchyoungerthanthatof thehostcaldera[Lipman
et al., 1976].Additionally,becausetheyaredeep-seated
bodies,
resurgentintrusionsare leastlikely to be affectedby structures
in underlyingrocks.
2. In Trans-PecosTexas veins commonlyshow more consistentorientationsthan dikes,possiblyfor severalreasons.ff
fluid pressurein hydrothermalveinsis lessthanmagmapressurein dikes,theveinswill be lesslikely to occupypreexisting
fracturesthat are not perpendicular
to the leastprincipalstress
[Delaneyet al., 1986]. Also, becausehydrothermalfluids are
muchlessviscousthan any magmas,they are more likely to
reorientto prevailingstresses
[Emermanand Marrett, 1990].
HVa•-R¾
• At.: 1Vlm-C•oz•c STRESS
Evot.u'nouA•U 1VL,,olv•AaaSM
13,549
Finally,veinsclearlyform in brittlerock:dikesmay be em- examplessuggeststhe origin of the scatter and the caution
placedwhile the hostintrusionis still partly liquid and less necessaryin interpretingthesedata.
ableto transmitregionalstresses.
Christmas Mountains dike swarm. A 4044
Ma dike swarm
Some uncertaintiesremain even for resurgentintrusions. in the ChristmasMountainsis one of the earliestclear palcoMagmapressurewithin theintrusioncouldinducea localstress stressindicators associatedwith igneousactivity and a good
field, analogousto thatof the SpanishPeaks.Magma pressure example of scatter around an east-northeastpeak (Figures4
would not affect veins formed after solidification of the intruand5). The dikesarerelatedto a smallcalderacomplexcentered
sion,however.Also,thecalderablockupliftedby theresurgent in the Christmas Mountains and to a nepheline-normative
intrusionmay be in part floatingon the intrusionand therefore gabbrothat precextedcalderaactivity;both are approximately
decoupledfrom the regionalstressfield. This possibilitymay 42 Ma [Henry et al., 1989; Henry and Price, 1989]. Dike
explain the random orientationof dikes within the Questa compositionsrange from basalt,petrographically
similar and
caldera, New Mexico, and distinct northwest strike of contem- probablyrelatedto the gabbro,to rhyolite,probablypart of the
poraneousdikesoutsidethe caldera[Lipman, 1983].
caldera system.
Ages •,r seven dikes, r,,,,,-,•,,t;,,o m,•r or the.Detro•raohic
DIKS•rr) Vsrq Or.m•^TiO•S I>tau•oCr•ozom •o••,
variation, range from 40.3 to 47.2 Ma [Henry et al., 1989;
Tms-P•os
T•
Laughlinet al., 1982].The 47 Ma dike is likely a feederto the
extensiveAlamo Creek Basalt and may not be part of the
Di•s and Vei• Before 32 Ma
D•es
•d
ve•
••
•twe•
48 •d
32 Ma •
ChristmasMountainsswarm[Henry et al., 1989].
The dikes,only a fractionof which are shownon Figure 5,
Tr•-
P•os Tex• •••tly
s•e e•t-no•e•t
to •st •i•e
4).
•e•
•ta •e repre•mative of •e maj• i•eom c•ters of
ß e region,mostof w•ch •e cflderas,•clude ex•pl•
•m
•
•e west• a•i•cic
•d •e eastern•alic •lm, •d
sp• n•ly •e entke t•e of pre-32 Ma activin. The 32 Ma
C•ati Mo•t•
c•dera [Cepe• and Hens, 1983], • •e
w•tm •lt, is •e yo•gest centerrepre•m•. •e Org•
Mo•tm
cald•a [Seager,1981], • sou•em New Mexico,
is geo•ap•cally •d chemically p•t of •e western •lt
•ig•e 4).
Ages assi•
• Fig•e 4 •e b•
on dk•t det•atio• on d•es • •e C•ati Mo•ta•
•d P•e C•yon
c•der•,
t•
P•s•o
volc•,
•
•e C•s•as
Mo•ta•
c•ters
of Tr•-P•os
Texas w•e
isolat•
•om
•ch
o•er, were active•iefly, •d were not •f•t•
by yo•ger
ß emal evenm[Hen• a• Pr•e, 19•; Hen• a• McDowell,
1986;Hen• et al., 1989]. U•at•
•es •e ti• to • over•l
calderamagmafismon •e b•is of field relatio•ps, pe•ography,•d cm•sifion. Ve• •e h•t• by restgrot •si•
of •e calderas•d •e •ical of opm-spa•-fill•g hy•o•emal
ve•
relat•
to •sio•.
Therefore, •
d•
•
ve• must have fo•
d•g
cald•a ma•afi•
or d•g
p••
c•l•g.
•e ages of ve• • •e Sh•t• silver disffict •d •e
Terl•gua merc• disffict,w•ch •e outsideof c•der•, •e
•e•
to • a•ut 32 •d 34 M• r•p•tively. •e Sh•ter
disffict is along •e sou•em m•g• of •e 32 Ma •ati
Moumm caldera;••ali•tion
•ere is probablyrelat• to
calderaign•us activin. •e ofig• of •e Terl•gua merc•
deposimis •cert•,
but •ey •e m•t l•ely relat• to 1•
i•us
activi• [Yatesa• T••son,
1959]. K-• agesof
•sio•
• •e •
•e mostly a•ut 34 Ma [Hen• et M.,
1986].
All dma •m
show •
do••t
e•t-n•ast
orientation
•ig•e 4), butcl•ly o•er od•tafi•
••
•
a dist•ct
a few centers. Most •atter
•o•d
•
•e pm•t
east-not,
and the rose diagram demonstratethe dominant east to east-
northeastorientation(Figures4 and 5). The completedike
swarm is exposedbecauseyoungercover is negligible.The
small but distinctnortherly and northwesterlypeaks on the
rose diagram almostentirely reflect the two long dikes south
and southeast of the Christmas Mountains.
The dike orientationsindicatethat the minimumprincipal
stress
(o•) wasnorthto north-northwest
andthatthemaximum
horizontalstresswas east-northeast,
parallel to the preferred
dike orientation.For reasonsdiscussed
more fully below,we
interpret
themaximum
horizontal
stress
tobeo•. Although
the
variation in dike orientations could reflect all the uncertainties
•sive •
[Hen• et d., 19•, 1989].A•itio•ly,
•
•
ve• •e ••
to have •e s•e age as •e host c•dera
•oughout •e regio• wi• •e p•sible excepti• of • •g•
Mo•tm
c•dera • discuss• •1ow. •t•l•
g•logic •
g••onological s•dy demo•at•
•at cald•as •d o•er
volc•c
exhibita wide rangeof orientations.
However,boththe map
st
p•; o•ers (e.g., •e •i•
Mo•t•
•
•g•
M•ta•) •e cl•ly b••l
wi• a no• or no•west •d
•
a•ition to a pm•ent •t •en• Ex•afion of repr•nmtive
citedabove,hostanisotropyis probablymostimportant.The
dikes were eraplacedinto Cretaceoussediment• rocks that
are not significantlydeformedhere but are intenselyfolded
andfaultedin north-northwest
trendsjust20 km to thesouthwest
[Erdlac, 1990]. Additionally,complexlydeformedPaleozoic
rocksof the Ouachita-Marathon
foldbelt occurat depthsless
than 1 km [Henry et al., !989]. A local stressfield resulting
fromintrusionof thegabbroor the siliciccalderamagmabody
wasprobablynot a factor,becausedikesmaintaintheirdominant
east to east-northeast orientation within the intrusive area.
The variation
in dike orientations
around the Christmas
Motretains
illustrates
theneedto knowthecomplete
distribution
of a dikeswarm.Examination
of onlya few dikescouldeasily
give a false impressionof stressorientations
duringtheir emplacement.Indeed, Laughlin et al. [ 1982], who dated the two
long dikes south and southeastof the ChristmasMountains,
interpreted
themto indicatemajorchanges
in stressorientations
at the time.
Quitman Mountains caldera. Dikes in the Quitman Mountains show a bimodal distribution(Figure 4). Dikes that cut
ring fracture intrusions and caldera fill volcanic rocks strike
east-northeastor approximatelynorth [Albritton and Smith,
1965]. Epithermalveins in the intrusionsstrikeeast-northeast.
By our criteria,the east-northeast
trendis mostlikely perpendicularto the leastprincipalstress,whichwouldbe northnorthwest.The sign/ficanceof the north trend is uncertain,but
it may be relatedto preexistingstructures
in Cretaceous
rocks
foldedduringLaramidedeformation.
The northstrikingdikes
are petrographicallysimilar to other intrusive rocks of the
QuitmanMountainscaldera;therefore,
theyareunlikelyto be
muchyounger.
13,550
H•rR¾ • AU.:Mm-Cn•ozoxc STatusEVOLUTiOn
nnv ]VL•oMATI{M
N
20%
Organ Mountains
36 Ma
22kin
106 ø
105
104ø
I
i
3Zo•
I
•,. I---'] El Paso
I
•
•
I!!1
Marble
Canyon
Quitman
Mountains
•-•If:•
36Ma
11km,
>31
'"k •;•i[ :,/T••: j•
•
'•:'.;"•
Eagle
Mountains
|
36Ma
/
"'x.
10km
!
•
Veins
incontemporaneous
intrusive
rocks
i:.i;• Dikes
incontemporaneous
intrusive
rocks
•
Dikes
and
veins
incontemporaneous
extrusive
rocks
within
calderas
++••'•
++ Dikes
incontemporaneous
extrusive
rocks
outside
calderas
•
Veins in Permian and Cretaceous sedimentary rocks
•
Dikes
inPermian
and
Cretaceous
sedimentary
rocks
N
Paisano-]'10%
36Ma
+++
?l•lkkm
,.
30ø-T
Ma
Ch•nati
II ••
32Ma Mountains
21km
{• ••
•
}
Christmas
Mountains •, 10%
115km
•20Yo /•
29ø +
+
0
t
+
•X 33Me
Pine
Canyon
•0%•
Terlingua / •
••
+
60 mi
'
'
•
'
0
'
',
IOOkm
0A•5•49
Fig. 4. S•e •eB• for d• •d vets, wi• age• t• s•e ]•g•, dung ma•ad• •tw•n 48 •d 32 Ma • TmnsP•os Texas.Mostd•e •d ve• •stems am ••
or adjacmtm •deras. S•e ba• am cons•ct• for 1• •te•s;
]en•s of
b•s ampr•on•
to •rc•ges of
p••]
stress
(s3)wasno• tono•-no•wesLMa••
p•cipal stress
(s=)was•mt•
to• east-not,st(seetext).
A possibleexceptionto the consistency
of north-northwest
Additional exan•les and one exception.Other, smalleror
occurred
at approximately
37 Ma. A group
incompletelystudieddike and vein systemsin Trans-Pecos leastprincipalstress
Texas show dominant east-northeast trends. Dikes associated of extensive,large-volume,silicic lavas,the BraeksRhyolite,
Trachyte,wereendlaced
with 47 Ma syeniticintrusionsin and southeast
of E1 Paso StarMountainFormation,andCrossen
from north-northwest
strikingfissures
strikenortheast(Figure3a) [AlbrittonandSmith,1965;Hoover at thistime,apparently
a brief episodeof
et al., 1988). The dikes have not been mapped in detail, [Henryet al., 1990].Thesecouldrepresent
extension,but such an event would clearly be
however,andareasof outcroparesurrounded
by upperTertiary east-northeast
basinfill that may cover many dikes. Syenitic,aplitic, and atypicalof thestressfield in Texasat thetime.
Posternplacement
rotationof thedikesaroundverticalaxes
pegmatiticdikes in the 37 Ma Marble Canyon(Figure 4)
intrusion of the eastern belt strike east-northeast[Price et al.,
is not a problemin Texas.The onlysignificant
deformation
to
1986], as do dikes and epithermalveins in the Solitario have affecteddikes is Basin and Rangeextension.In Texas,
(Figure4), a 38 Ma laccolithandcaldera.
total extensionwas less than 10% as shownby the geometry
HENRY
ETAL.:MID-CENOZOIC
STR•$E¾OLI•ON
ANDM•OMA'rlsM
.-
o o7 "' %•,---
•' ///
•
xß
/'/
-/
' '
13,551
40.,4
+0.7• •
Big
Send
•
National
Park
j
ounua
i
0
"
5
I
I
0
J5
I ,
10 mi
I
• I
10
15 km
/
/
(•A•5•
Fig. 5. Dikesin the Christmas
Mountainsarea,southern
Trans-Pecos
Texas.LighterlinesdenoteEocenedikesrelatedto the
Christmas
Mountainsgabbro(shaded)andChristmas
Mountainscalderacomplex(hachured).
Only a representative
fractionof the
211 measured
dikesareshown.HeavylinesdenoteMiocenedikesrelatedto east-northeast
extension.Olderdikesshowa rangeof
orientationswith a maximumat east-northeast.
Youngerdikes strikeuniformlynorth-noahwest•Locationand rose diagramsof
dikeorientations
areshownin Figures4 and7. AgesfromHenryetaL [1989]andLaughlinet aL [1982].
of extendedrocks within both horstsand grabens[Henry and
Price, 1986]. Strike-slipdisplacement
thatcouldgeneratesignificantverticalaxisrotationoccurredalongstructuresoblique
to the east-northeast
extension[Muehlberger,1980]; however,
the total amountof suchdisplacement
was far too small to
haverotatedsignificantareas.
Kj
Q
Interpretationof east-northeast
orientationof o•. The
preferredeast-northeast
orientationof dikesandveinsin Trans-
Pecos
Texasindicates
thattheminimum
principal
stress
(o•) was
north-northwest
andthatthemaximum
principal
stress
(o•) was
in theplaneof thedikesandveins.However,o• couldrange
from
east-northeast
to vertical.
The former
stress field
is
Q
commonlyconsidereda strike-slipenvironment,whereasthe
latter is that of north-northwestextension.Became faulting
accompanying
magmatismwasnegligible,interpretation
of the
trueenvironment
is equivocal.As discussed
below,we interpret
Q
83
o• to be east-northeast
because
of (1) evidence
for minor,
contemporaneous
strike-slipfaulting,(2) by comparisonwith
the stressfield during Laramide folding that immediately
precexled
magmatism,
andO) by analogywith stressorientations
in Arizona and New Mexico.
Two setsof dikesat Glenn Draw in Big BendNationalPark
illustratetheevidencefor strike-slipfaulting(Figure6) [Price
andHenry, 1988]. At thislocation,one dike setstrikesapproximatelynortheast.
The othersetformsa west-northwest
trending
en echelonpatternin which individualdike segmentsstrike
east-northeast.
We interpretthesedikesto fill mode 1 fractures
along a left-lateralshea•zone. En echelondikes along complementarynortheasttrendingright-lateralshearsoccurat two
locationswithin 8 km of the area depictedin Figure 6. The
agesof the dikesat GlennDraw are not well established.
The
0
0.5
I
,
0
0.5
,
1.0 mi
I
•
,
1.0
•
1.5 km
Quaternary
deposits
Tertiary
dikes
Javelins
Formation
Cretaceous
Aguja Formation
basaltis compositionally
like the48-32 Ma rocksanddistinctly
unlikethemoreprimitivebasaltsemplacedafter31 Ma [Price
6. Geologicmapof dikesin Glmn Draw area,Big BendNational
and Henry, 1988;Jamesand Henry, this issue.]An apparent Fig.
Park, modified from Maxwell et al. [1967]. Individual dikes in west-
K-Ar ageof 57 Ma (.•_5.6
Ma) probably
reflects
excess
argon
northweststriking,en echelonsystemstrike east-northeast,
parallel to
asno Cenozoicigneousrocksare thatold in Trans-Pecos
Texas probablemaximumprincipalstress.
13,552
Iq•¾
• st_: Mm-C•ozo•c
S'm•ss EvoLm•on ^NX>
Mnom,'mM
[Henry et al., 1986]; excessargonhasbeenfoundunequivocally
in similar basaltsin the Big Bend area [Harlan et al., 1986].
Reliabledateson chemicallyandpetrographically
similarrocks
in the Big Bend area are mostly around 34 Ma [Henry and
03 waseast-northeast.
Thedikesareaccompanied
by a series
of small-displacement
normalfaults(Figure8). Dikes andfaults
were probablybroadly contemporaneous.
Faults mostly cut
dikes,but a few dikes appearto have intrudedalongfaults.
McDowell, 1986].
Paleostressanalysisfrom fault and slickenlineorientations
Orientationsof folds, thrustfaults, slickenlines,stylolites, of 31 faultsusingthe methodof Angelier [1979] indicatesthat
calcite twin lamellae, and other kinematic indicators demon- o3 plungedgentlyS63øWand that c• wasnearlyvertical
stratethatc• waseast-northeast
ando3vertical
duringLaramide (Table 1).
folding that precededmagmatism in Texas [Berge, 1982;
Moustafa,1988;Erdlac, 1990]. The similarityin orientationof
maximum horizontal stressand the lack of a significanttime
gap betweenfolding and magmatismsuggestthat the east-
The age of the dikes and thereforeof initial east-northeast
extensionis basedlargelyon a singleK-Ar ageof 31.4 -l-0.7
Ma on biotiteof thehostgranite[Gregory,1981].The dikesare
f'me-grained,
porphyriticvarietiesof thegraniteandclearlythe
northeast
direction
continued
to be o•, although
theminimum same age. Dates of 30.2 + 0.7 and 30.6 + 0.7 Ma on alkali
from the ashflow tuff derivedfrom the
principal
stress
(o3)rotated
fromverticaltonorth-northwest. feldsparphenocrysts
The analogy to stressorientationsin Arizona and south- SanCarloscalderalendsomesupportto 31 Ma age.However,
western New Mexico is less direct, but the data are more
field relationsdo not constrainthe relativeagesof the granite
definitivein thoseareas.Heidrick and Titley [ 1982] alsofound and ash flow tuff. Additionaldating of the graniteand pareast-northeastpreferred orientation of dikes and veins. ticularly the dikes is warrantedto constrainthe age of the
Additionally, they found conjugate shears arranged sym- stresschange.
metrically
around
thedikesandveinsthatdemonstrated
thatc•
washorizontalandapproximatelyN70øE.
Bofecillos Mountains. East-northeastextension was un-
equivocallyestablished
by 28 Ma basedon thenorth-northwest
Althougha stressfield with c• ando3 horizontal
is com-
strike of dikes associated with the 28 Ma Bofecillos Mountains
monly considereda strike-slipenvironment,we do not refer to
it in this way. Strike-slipfaulting,in fact, faultingof anykind,
was negligible during magmatismbetween48 and 32 Ma in
Texas. The only faulting was related to igneousdomingand
calderacollapse.In contrast,strike-slipfaultingwas relatively
volcano(Figure7). The subsidiary
northeast
to eastpeaksmay
reflectdikeswhoseorientations
were controlledby structures
in underlyingrocks.The 28 Ma volcanicrocksin theBofecillos
Mountainsare underlainby 32 Ma and older volcanicrocks
thatlikely haveeast-northeast
strikingjoints.Therefore,the 28
common
bothduringpreceding
Laramide
compression
(o•east- Ma eaststrikingdikesmay reflect the influenceof basement
ratherthanof preexisting
jointsin the exposedhost
northeast
ando3 vertical)andduringsubsequent
Basinand structures,
Rangeextension
(o• verticalando3east-northeast)
asa result rocks, as discussedabove.
of reactivation
of west-northwest
basement
structures under
thosestressfields [Muehlberger,1980].
Relative stressmagnitudes. The considerablescatterof dike
and vein orientationsaroundthe east-northeast
pealcsuggests
that the difference in magnitudebetweenthe two horizontal
stresseswas small. Delaney et al. [ 1986] considerthis characteristicof tectonicallyinactiveregions,whichseemsconsistent
with the lack of faultingin Trans-PecosTexasbetween48 and
32 Ma. Nevertheless,despiteits relative inactivity,the area
still had a distinctstresspatternthat governeddike andvein
orientations.
Amount of northerly extension.The total mount of northerly extensionrepresentedby the dikes and veins is small
throughout
Texas.Bothdikesandveinsarerelativelythinand
only locally abundant.Dikes rangeup to about10 m thickbut
probablyaverageabout2 m. Veins are uniformlylessthan a
few meters thick. Even where dikes are most abundant, for
example,westof the ChristmasMountains,totalextensionwas
no more than a few percent.Areasbetweendike swarmsmay
haveundergoneno extension.
Dikes and VeinsAfter 32 Ma
Dike orientationsand fault kinematic data indicatea change
to o3 east-northeast
andc• vertical,probablyby 31 Ma and
definitelyby 28 Ma. This changemarksthe beginningof eastnortheast extension, which characterized early extension
throughout
theBasinandRangeprovince[Zobacket al., 1981].
SierraAzul, Chihuahua. The earliestevidencefor the change
occursin the SierraAzul of northernChihuahua,whichis part
of the San Carlos caldera (Figures3 and 7) [ChucMa, 1981;
Gregory, 1981]. Dikes in a graniticintrusiontheresltike dominantlynorth-northwest(Figures7 and 8), which indicatesthat
Basin and Range basalts. The youngestigneousactivity
in Texas consistedof widespreadbut volumetricallyminor
alkalibasaltsand hawaiiteseraplacedbetween24 and 17 Ma
(Figures3d and 7). The rocksare preservedmostlyas dikes,
but lavasfed by thedikesoccurin a few areas.Throughout
the
region,thedikesdominantlystrikenorth-northwest,
indicating
continuedeast-northeastextension.Lengths and orientations
of all known dikes were determined at two locations: the Rim
RockdikeswarmneartheRio GrandeanddikesnearTerlingua
westof Big BendNationalPark (Figure7).
The Rim Rock dike swarm consists of at least 480 dikes
totaling70 krn in lengthandrangingin agefrom 24 to 17 Ma
[Dasch et al., 1969; Henry and Price, 1986]. The dikes are
generallyvertical and intrudeCretaceous•entary
rocks
and38-32 Ma Tertiaryvolcanicandvolcaniclastic
rocks.The
subsidiarywest-northwest
trend includesmany of the oldest
dikesof theswarmandparallelsa majorPrecambrian
basement
trend[Muehlberger,1980].Dike emplacement
alongthistrend
probablyreflectsinitialopeningof thebasement
structure
under
east-northeast
extension[Henryand Price, 1986]. As extension
continued,thenorth-northwest
trendopenedandbecamedominant. The west-northwest
structureis otherwisenot apparent
in the Cretaceousor Tertiary host rocks; therefore, dike
eraplacementmay have been controlled by the basement
structureratherthanby preexisting
_joints
in the exposedhost.
North-northwest
strikingBasin and Rangefaults throughout
Texascommonlystepover alongshortwest-northwest
segments
that reflect the basement structure.
A smallerswarm,consistingof 35 dikestotalling5 km long,
occurssouthwest
of theChristmasMountains(Figures7 and5).
Three dikes have been datedbetween24 and 20 Ma [Henry et
al., 1986]. The consistentnorth-northwestorientation of these
dikes contrastswith the generally east-northeastbut more
HENRYBTAL.' M.lZ)-CI•OZOlC
STRESS
EVOLUTION
ANDMAOMATI•M
106 ø
105 ø
104 ø
i
i
13,553
I
<_31 Ma
El Paso
Veins
incontemporaneous
intrusive
rocks
x,
lii!ii!iiiil
Dikes
incontemporaneous
intrusive
rocks
r,-+ + + +• Dikes in contemporaneous extrusive rocks outside calderas
I
] Dikes
inCretaceous
and'Tertiary
rocks
•
BasinandRangenormal
faults,_<24 Ma
31 ø -I-
M
oDuan•'•
ns%'•'•
N
Rim Rock
24-17 Ma
70km
ß
-I-
30 ø +
+
BlackGap
N
",-er,in,ua
20%
24-20Ma
(
5km,
Bofecillos
+•I .
28Me
'•'•
'•+-•_•
•32km
J
29 ø -i-
0
I
60 mi
'
'
I
'
'
0
31Me,
',
IOOkm
QA 15150
Fig. 7. Dikes,veins,andfaultsrelatedto post-31Ma magmatism
in Trans-Pccos
Texas.Fourrosediagrams,constructed
from
percentages
of total strikelengthas in Figure4, showorientations
of majordike swarms.All features,including24 Ma dikesin
BlackGapandthe DavisMountainsstrikedominanQy
north-northwest,
indicatingthatleastprincipalstresswaseast-northeast.
Subsidiary
west-northwest
andeast-northeast
peaksreflectreactivation
of olderstructures
(seetext).Calcicras
dominandy
predate
extension
butarerarelycutby majorBasinandRangefaults.
variable
orientation
of the 42 Ma
dike swarm.
A few
Basin
andRangedikescut dikesof the older swarm.
Relative stressmagnitudes.Dikes emplacedafter 31 Ma
show a much narrower range of orientationsthan do dikes
eraplacedbeforethen.This consistency
suggeststhat the two
horizontalstresses
haveconsiderably
differentmagnitudes.
The
which,in ram, seemsconsistent
with emplacement
of thedikes
duringactiveextension.
Timing of Basin and Range faulting. Basin and Range
faulting, which was exclusively high-angle,began slightly
before 24 Ma.
Volcanic
rocks of the 28-Ma
Santana caldera
and Bofecillos volcano show no changein thicknessacross
valueof f (0.768wheref = (o•.-o•)/(Ol-O•);
Table1) calculated Basin and Rangefaults [Henry and Price, 1986]. Basaltlavas
from fault and slickenline data from the Sierra Azul also indiof the 24 Ma suiteare interbeddedwith lower parts of basin
catesthato•.ando• weresignificantly
different.
Delaneyet al. fill in severalgrabensbut are never at the baseor below basin
[1986] characterize
this situationas beingtectonicallyactive, fill. Thusfaultingmusthave startedsufficientlybefore24 Ma
13,554
•¾
E'rAL.:MtD-CI•ozoIc STRESS
EVOLUTION
AND]V•,OMATISM
New Mexico
N
Dike and vein orientationsin the OrganMountainscaldera
of southernNew Mexico (Figure4) illustratethe complexity
that can occur. Veins dominantlystrike approximatelyeast
[Dunham,
1935;Seager,1981];we consider
these6
to be the
mostreliableindicator
of paleostress
andtherefore
indicate
o3
to be north.Many dikesalsostrikeeast,but mostdikesstrike
northwest
[Seager,1981]. Conventional
K-Ar agesof volcanic
and intrusive rocks of the caldera, including one northwest
strikingdike, are32 to 33 Ma [LoringandLoring, 1980,Seager,
1981]. New Ar•ø/Ar• ageson ash flow tuffs of the Organ
caldera,includingunits previouslydatedas 33 Ma, are 35.6
and36.1Ma [Mcintosh,
1989].Theorganba.[holith,
the
resurgentintrusionof the calderathat hoststhe veins,hasnot
beenredated.However,given the brief periodof activityof
mostcalderas,
thebatholithandtheveinsaremostlikely36 Ma.
Newcomeret al. [1983] interpretedthe northwestdikesto
indicatenortheasterly
extension,beginningby 32.5 Ma, the
K-Ar age of one of the dikes [Loring and Loring, 1980].
However,the dikeshavenot beenredated,andthenew Ar•/Ar •
datasuggestthat they may be older.A 32.5 Ma ageis within
Fig. 8. Stereonet
plotof small-displacement
faultsandslickenlines
(curves analyticaluncertaintyof the beginningof extensionin Texas,
andsolidcircles)andpolesto dikes(opensquares)
andveins(solidsquares) whereas36 Ma is distinctlyolder.More critically,the dikesdo
OA 15154
withintheSierraAzul graniteof the SanCarloscaldera,Chihuahua.
Faults
notoccupymode1 fractures;
theyarereported
to havebe•
werecontemporaneous
withdikesofthegranite,
whichis31Ma [Gregory, emplacedwithinnorthwest-striking
shearzones[Seager,1981].
1981;Chuchla,1981]. Quantitativeanalysisof the fault dataindicatesthat
extensionregardleastprincit• stress
(os)wasS63øWandthemaximum
princit• stress Thereforethey do not indicatenortheasterly
(o•) wasapproximately
vertical(Table1). Similarly,dike andvein lessof their age.
orientations
indicatethatthe leastprinci• stresswaseast-northeast
(see
East-northeast
extensionclearlydid beginin theRio Grande
alsoFigure7).
rift in New Mexico at about31 Ma (Figure 9b) basedon dike
orientationsand domino-stylefaulting [Laughlinet al., 1983;
Chamberlin, 1983; Aldrich et al., 1986]. Also, substantialeastfor basins to form and accumulate a few tens of meters of
northeast
extension
occmxed
at 26 Ma
in the Latir
volcanic
coarsesediment.The beginningof significantfaulting may field in northernNew Mexico [Lipman, 1983]. We find no
have laggedthe stresschange,which we considerthe more evidencefor east-northeast
extensionas early as 40 Ma as
fundamental
event,by as muchas 7 m.y. Nevertheless,
both proposed
by Elston[ 1984]for theMogollon-Datilvolcanicfield.
faults and dike orientations indicate east-northeast extension.
Arizona
STRESSORIENTATIONSIN THE SOUTHWE•ERN UNITED STATESAND
NORTHERN Mumco
and Southwestern
New Mexico Laramide
RehrigandHeidrick [1976] andHeidrickand Titley [1982]
examinedin great detail the orientationsof dikes and veins
associated
with Laramideplutons(75-50 Ma) of Arizonaand
A preliminarylookat theevidencefor paleostress
in adjacent southwesternNew Mexico. They found a dominant eastareasof the southernCordillera to comparewith the stress northeastorientationto purely dilatantjoints throughoutthis
evolutionin Texasis worthwhile.
Comprehensive
analyses
have region (Figure 9a). Additionally, they found two sets of
been done in southwesternNew Mexico and Arizona [Rehrig
and Heidrick, 1976; Heidrick and Titley, 1982], in the Rio
fractures,N60øE and N80øE, with horizontal slickensidesthat
representconjugateshear.From all thesedata,Heidrick and
Grande
rift [Aldrich
et al., 1986],andmorebroadly
in the Titley [1982] concludedthat the maximum principal stress
to that;
southwestern
United States[Best, 1988]. Additionally,sparse wasN72øE andthe leastprincipalstressperpendicular
data are available for northwestern Mexico.
this stressfield persistedto at least 56 Ma and possiblyto
50 Ma. Furthermore, they attributed this stresspattern to
similarlyorientedFarallon-NorthAmericaplate convergence,
an interpretation
with whichwe agree.
Rehrig and Heidrick additionallydeterminedthat upper
TABLE 1. StressOrientationsCalculatedFrom SlickenlineData,
SierraAzul, Chihuahua(31 Ma) for 31 Faults
Tertiary(<30 Ma) epithermal
veinsin Arizonadominantly
strike
Orientation
north-northwest
(Figure9b). They interpretedthis to indicate
east-northeast
extension.Althoughthe paucityof magmatism
N7øW
betweenabout50 and 30 Ma in Arizonaprecludesdetermin77 ø
ing the exacttime of change,the stressevolutionthereappears
S28øE
12 ø
S63øW
5ø
(o2-o3)/(Ol-03)
0.768
to be identical to that in Texas.
Southwestern
United States
Best [ 1988] interpreteda more complexevolutionof stress
fromdikeorientations
in partof thesouthwestern
UnitedStates
HENRYh'TAL.' lktlD-Cl•ozoIc STR•$ EVOI.IrHONANDMAOMATISM
o
o
13,555
oo
•'
o%
Vl
z
•
E
o
o-
z
•
•
½
13,556
HENRY
ETAL.:M.ID-CENOZOIC
STRaSS
EVOLUTION
ANDMAOMATISM
from Nevadato Texas.He interpretedthe changein leastprin- the Basin and Rangebasaltsof similar age in Texas,occupy
cipal stressfrom more northerlyto easterlyto have occurred identicalstructuralpositions,and indicate a similar tectonicdiachronously,
from as old as 31 Ma in Texas(basedon our magmaticsetting.
data) to 26 Ma or even 18 Ma farthernorth and west.However,
Best noted that dike orientations
seemed to indicate continued
northerlyleast principalstresswhereasextensionin contemporaneous
metamorphic
corecomplexesin southernCalifornia,
Arizona, and Sonorawas uniformlyeast-northeast
(Figure9b)
[Wust, 1986]. Also, Delaney et al. [ 1986] analyzedthe orientationsof dikesandrelatedjoints to indicatea changein least
principalstressfrom N34øW to N80ø-90øEbetweenapproximately 31 and 27 Ma in southernUtah and northernNew
Mexico (Figure9a, Ship Rock, andFigure 9b, MexicanHat).
This area is within the ColoradoPlateauso its applicationto
the Basin and Range province is uncertain.Nevertheless,
someof Best's data were also from the plateauimmediately
adjacentto the areaexaminedby Delaneyet al.
The resolutionto this discrepancy
may lie in the dataused
by Best[1988]. To minimizethe influenceof preexisting
joints
formedin older stressfields, he useddikeseraplacedinto host
rocks that were only slightlyolder than the dikes.However,
nearly half of the resultant27 occurrences
consistedof only
one to threedikes.Without knowing the detailsof eachoccurrenceandnotingall the uncertainties
listedabove,we suggest
that the small number of dikes measuredin some areasmay
not allow a reliable determinationof palcostress.
Instead,the
The available
data in northern
Mexico
thus indicate
east-
northeast
o• at leastto about37 Ma and east-northeast
os
beginningby about 31 Ma. The abundanceof evidencefor
east-northeast
extensionbetween31 and28 Ma suggests
thatit
may have begunthen. At the least, the data allow a similar
stresshistory as found to the nor& Clearly, more thorough
study is ne.e.
xted to understandstressevolution in northern
Mexico.
CENOZOIC STRF3S EVOLtrHON IN THE SOUTHERN CORDILLERA
During LaramideDeformation
The comparisonof datafrom the southwestern
United States
andnorthernMexico suggests
a similarstresshistorythroughout
this region. Laramide folding occurredat different times in
Arizona and Texas but largely precededmagmatismin both
areas [Drewes, 1978; Henry and McDowell, 1986]. The socalled Laramide plutonsof Arizona mostly postdatemajor
folding,althoughminor,laterfoldingmayhaveoccurredlocally
[Drewes, 1978]; thus Laramide deformation there occurred
largelybefore70 Ma. In Texas,Laramidefoldingoccurredin
sum of data mentionedhere is consistentwith a changein least the early Eocene[Wilson, 1971], endedat about50 Ma, and
principal stressfrom north-northwestto east-northeast
about precededall magmatism.The 47--48 Ma intrusiverocksin the
E1 Paso area crosscut Laramide
folds and are themselves
30 Ma throughoutthe southwestern
United States.
undeformed[Hooveret al., 1988]. Basaltlavasof the sameage
in Big Bend unconformablyoverlie folded Cretaceousrocks.
Northern Mexico
Possiblymagmas,if present,could not penetratethe crustin
environment.
Our preliminarydata on the evolutionof stressin northern a stronglycompressional
Mexico indicate a similar stress evolution to that in the southDespitethe differencein timing of Laramidefolding,stress
westernUnited States[Price, et al., 1988; Drier, 1984]. Epi- orientationsduringfolding in Texasand Arizona were similar.
thermalveinsat themajorminingdistrictof Tayoltita,Durango, Transport directions, orientationsof folds, thrust faults,
stylolites,calcitetwinlamellae,andotherkinematic
west of the Sierra Madre Occidental,strike dominantlyeast- slickenlines,
demonstrate
thato• waseast-northeast
andosvertical
northeast(Figure9a) [Smithet al., 1982]. Dating of hostrocks indicators
and vein adularia indicates that vein formation occurred at
[Drewes, 1978;Berge, 1982;Moustafa, 1988;Erdlac, 1990].
40 Ma (C. D. Henry and F. W. McDowell, unpublished
data,
1990). A belt of 3743 Ma intrusionsnear Monclova, Coahuila,
During Mid-CenozoicMagmatism
alsoindicates
east-northeast
s•(Figure9a).Thebeltitselfstrikes
east-northeast,and dikes related to the intrusions strike east-
northeast[Sewell,1968;R. E. Denison,unpublished
data,1990].
Examplesof east-northeast
extensionin northernMexico
are widespread both east and west of the Sierra Madre
Occidental,a relatively unextendedblock [Henry, 1989]. In
general,northernMexico is a little recognizedcontinuationof
theBasinandRangeprovincecharacterized
by north-northwest
striking basins and ranges [Stewart, 1978; Henry, 1989].
Severalmajor miningdistricts,includingSantaBarbara-Parral
[Grant and Ruiz, 1988] and Guanajuato[Gross, 1975], are
characterized
by generallynorthwest-north
strikingveinsystems
andarereliablydatedat between31 and28 Ma (Figure9b). At
Guanajuato, the veins occupy contemporaneouslyactive
northwest-strikingfaults [Gross, 1975] that indicatesubstantial
northeast extension.
Other evidence for east-northeast extension includes a 28 Ma
Stressorientations
duringmagmatismimmediatelyfollowing
Laramidefolding appearto be similar throughouta transect
from southernArizonato Texasandlikely in northernMexico.
The maximum principalstresswas east-northeast,
as it was
duringfolding,but the minimumprincipalstressrotatedfrom
vertical to north-northwest. Note, however, that because
Laramidefoldingoccurredmuchlaterin Texasthanin Arizona,
o3wasnorth-northwest
in Arizonaatthesametimethatit was
vertical in Texas. Nevertheless,throughoutthis region before
about31 Ma, the maximum principalstressappearsto have
been east-northeast.In Texas before 31 Ma, both the western,
alkali-calcic and the eastern, alkalic belt show similar east-
northeastdike and vein orientations,indicatingthat both belts
wereexperiencingthe samestressfield.
A distinct
change
in thestress
field,to oseast-northeast
and
o• verticalor east-northeast
extension,
occurred
throughout
the
north-northweststriking dike swarm in Sinaloa, west of the
SierraMadre Occidental[Henry and Fredrikson,1987;Henry,
1989]. Near Nazas, eastof the SierraMadre, Aguirre-Diaz and
region. Where best dated in Trans-PecosTexas, this change
occurredat least by 28 Ma and possiblyabruptlyat 31 Ma.
The time of changeis less constrainedin other partsof the
McDowell [1988] found 23 Ma alkali basaltsrelated to north-
southwestern United States and in northern Mexico, but the
northweststrikinggrabens.The rocksare chemicallysimilarto
dataallow the changeto have occurredat 31 Ma also.
H•_,,m¾
m' ^,-: Mm-C•ozom
S'm•s EvoumoN ^NI• MAO•AU'•SM
13,557
Vizcoino
Son Jose
Los Angeles
150
E I00
o
>,
,50
0
,.,
0
50
100
150
•
I
I
I
0
50
100
150
i
200
0
50
100
150
Velocityeast ( mm/o)
•)$6-42 Ma A42-50
Ma •50-59
Ma
Fig. 10.Farallon-North
American
plateconvergence
directions
andrates[Stock
andMolnar,1988].Locations,
whichspanthe
sonthem
Cordillera,
areSanJose,mouthof Gulfof California;
Vizcaino,VizcainoPeninsula,
BajaCalifornia;
LosAngeles,
Los
Angeles,
California.
Agesspantimeof mostactivitydiscussed
here.Ellipses
for Vizcaino
plotindicate
uncertainties
determined
by StockandMolnar.
ORIGIN OF THE STRESSPATTERNS
The similarityin stresspatternsthroughout
sucha wide area
requires.aregionalcause,which we suggestis relatedto plate
interactions.
In particular,
theeast-northeast
maximumprincipal
stressis parallelto Farallon-NorthAmericanplateconvergence
at the time (Figure 10) [Stockand Molnar, 1988]. The data in
Figure 10 spanthe southernCordillerafrom aboutthe TransMexican
volcanic belt to the southern United States and most
of the time of earliermagmatismdiscussedhere. Note that the
dike and vein orientationsand inferred maximum principal
stressareparallel to the convergence
direction.
Similarfracturepatternsandstressfields(i.e., perpendicular
to the continentalmargin and parallel to convergence)are
commonin convergentmargins[Nakamuraand Uyeda, 1980;
Hancockand Beyan, 1987;Feraud et al., 1987]. Additionally,
maximumhorizontalstressis commonlyparallel to absolute
plate motions [Zobacket al., 1989]. Although the origin of
broad-scalestressfields is beyond the scopeof this paper,
proposedmechanismsthat seem likely include sheartraction
at the baseof the lithospherethat is drivingor resistingplate
motion [Zobacket al., 1989] or shearbetweenthe subducting
andoverridingplates[McKenzie,1969;Nakamuraand Uyeda,
1980].
The only unusualaspectof the stressfield in Texas before
31 Ma
is its existence so far inboard from the continental
margin. We suggestthat this reflects the commonlycited
shallow subductionat the time [e.g., Lipman et al., 1971;
Coney and Reynolds, 1977]. The stressfield in Texas before
31 Ma corresponds
to a variationof case 1 of Nakamuraand
Uyeda [ 1980], in which the maximum horizontalstressremains
parallelto convergence
from arcto backarcregionsbut changes
from(I• to02.Nakamura
andUyedasuggested
thatthisreflects
gradual landward diminution of compressivestress. The
convergenceslowedmost significantlybetween56 and50 Ma
and that the convergencedirection rotated clockwise, from
northeastto east-northeast.
Theynotedthatthe50 Ma slowdown
doesnot correspond
to the commonlycited40 Ma end of the
Laramideorogeny[Coney, 1972] and that clockwiserotation
was oppositeto the suggestedcounterclockwise
rotationof the
orientationof crustal shortening[Chapin and Cather, 1983].
Both observations of Stock and Molnar [1988] coincide
exceptionallywell with the timing andorientationof Laramide
deformationin Texas, however.Paleontologicdata [Wilson,
1971] and the agesof undeformedigneousrocksthat immediatelyfollow foldingindicatethatLaramidedeformation
ended
by approximately50 Ma. Additionally,orientationsof folds,
faults,stylolites,
andcalcitetwin lamellaeindicatethato•
rotatedfrom northeastto east-northeast
duringLaramidefolding [Berge, 1982; Price et al., 1985]. Dickinsonet al. [1988]
interpreteda diachronous
end to Laramidedeformationas late
as 35 Ma in New Mexico. However, their data are inconsistent
with its unequivocal
endat 50 Ma in Texasimmediatelysouth
of New Mexico. Thus, on the basis of Stock and Molnar's
data, we f'md a good correlationbetweentiming and orientation of Laramidedeformationand rate and directionof plate
convergence.Furthermore, we suggestthat the stressfield
between50 andabout30 Ma continuedto reflectconvergence
but that the slower convergenceresulted in a lower stress
magnitudeinsufficientfor significantdeformationto continue.
Our dataalsoprovidesomeconstraints
on currentmodelsof
the origin of mid to late Cenozoicextensionin westernNorth
America. These modelsemphasizeeither plate interactions
[Atwater,1970;GlaznerandBartley, 1984;Engebretson
et al.,
1984]or gravitational
collapseof oven/fickenecl
crust[Wernicke
et al., 1987]. Extensionmay havebeguncontemporaneously
throughoutthe southernCordilleraandat nearlythe sametime
as collision of the East Pacific Rise and palcotrenchabout
maximum
horizontal
stress
mayhaveremained
(I• overa wide 29 Ma [Stock and Molnar, 1988]. This collision marked the
zone perpendicularto the continentalmargin in the southern beginningof the end of convergence
over muchof the western
Cordillerabecausethezoneof couplingbetweentheshallowly marginof theNorthAmericanplate.Additionally,the areaof
subducting
slabandthe overridingNorth Americanplatewas extension in Trans-Pecos Texas does not coincide with areas
so broad.
of likely crustalthickening.Laramidefolding was limited to
The dataof StockandMolnar [ 1988]alsobearon thetiming, the westernmargin of Texas, along the Rio Grande,and the
origin, and style of Laramidedeformation.They found that volume of preextensional
magmafismseemstoo small to have
13,558
HENRY•r AL.:/VI•-C•ozoIc S'm•s EVOLUtiON
ANDMAOM•'r•sM
induced much crustal thickening. Thus the data seem more
supportiveof a mechanismof extensionrelatedto the end of
convergenceand the changein type of plate margin than to
collapseof overthickened
crest.The possibilitythat extension
begannearly contemporaneously
over sucha broad area also
suggeststhat it is not solelyrelatedto triplejunctionmigration alongthe continentalmargin[Ingersoll,1982;Glaznerand
Bartley, 1984].
T•'romc
Sm'mqa OF Tms-P•cos
Mno•SM
Synthesisof regional age patternswith paleostressand
geochemicaldata [Jamesand Henry, this issue]from Trans-
Pecos Texas provides a clear picture of the evolution of
magmatism there: from a continental volcanic arc through
32 Ma to regional(Basin and Range)extensionafterward.
Briefly, the evidencefor a continentalarc settingup to
32 Ma includes(1) theregionalagepattern,wheremagmatism
in Texasis part of a geographicandtemporalcontinuumwith
activityalongthe westcoastthat is clearlysubduction
related
(Figure 2); (2) the paleostress
data, which we interpretto be
related to paleoconvergence(Figures 4, 9a, and 10); and
(3) geochemicaldata that indicate a continentalarc setting
[James and Henry, this issue]. The geochemicalevidence
includes(1) the presenceof differentiationsuites,more alkalic
than, but otherwise similar to those of typical continental
asthenospheric
sourceswith no suggestion
of an arccomponent
[JamesandHenry, this issue].
Thesedataandinterpretations
mayprovidesomeconstraints
on the historyof plate interactionsin the southernCordillera,
which is poorly constrainedby plate kinematic models
[Severinghaus.
and Atwater, 1990]. Severinghausand Atwater
modeledthe thermalevolutionof the subductingslab beneath
North America to determinewhen it becamerheologically
incapableof seismicity.The same model also suggeststhe
relative capability of the slab to generatemagmatismand
transmitstress.By the criteriaof Severinghaus
and Atwater,
arc magmatismprobably shouldnot have been occurringin
Texasat 35 Ma. The clearpresence
of arcmagmatism
in Texas
at that time suggestseither (1) that arc magmatismcan occur
evenafterthe slabis seismicallyinactiveor (2) thatsubduction
geometryis insufficientlyknown for the southernCordillera,
assuggested
by Sevefinghaus
andAtwater.
LNTERPRETATION
OF ANCIENT R•om>
volcanic arcs; (2) evolution of the differentiation suitesfrom
Finally, we note that our data have implicationsfor interpreting older tectonic settings.Pre-31 Ma igneousrocks of
Trans-Pecos
Texascouldbe interpretedto be anorogenic,
given
their lack of significant,coevaldeformationand similarityin
compositionto what have been called anorogenicgranites
[Anderson,1983]. Nevertheless,the relativelysubtleevidence
of dikeandveinorientations
indicatethatpre-31Ma magrnatism
relativelyevolved,mantle-derivedmafic rocksby fractionation
occurred
and variable crustal assimilation; and (3) arc trace element
convergentmargin.It is becomingincreasinglyapparentthat
alkalicrocksare quitecomfortablein platemarginor orogenic
settings [Whalen et al., 1987; Reagan and Gill, 1989],
includingstronglycompressional
ones[Halliday et al., 1987].
On the otherhand,magmatismin Texaswas precededand
followed,by only a few million years,by majorcompressional
and extensionaldeformation.Indeed,magmatismwas continuousinto the episodeof extension.In older igneousterranes,
signatures
(low Nb andTa, high largeion lithophileelements).
Pre-31 Ma Trans-Pecos
magmatismis simplya more alkalic
version of a typical continentalvolcanic arc. These characteristicsapplyacrosstheprovince:to thewestern,alkali-calcic
and eastern,alkalic belts, to peralkalinerocksin both belts,
and to silica-undersaturated
differentiation
suites that occur in
the eastern belt.
The beginningof regional,east-northeast
extensionat about
in a continental
volcanic
arc related
to a distant
resolution of such small time differences could be difficult,
31 Ma is accompanied
by a two-stage
changein composition and magmatismcould be interpretedto be contemporaneous
of eruptedmagmas.Initially, between31 and 28 Ma, erupted with eitherof the deformations.
As an example,Barker [1977]
rockswere bimodal.Althoughthe rocksdisplaywithin-plate initially suggestedan analogybetweenTrans-PecosTexasand
or rift trace elementsignatures,they are intermediatein some the East African rift. He did so on the basisof similarityof
geochemicalmeasuresof arc versus continentalrift environ- rock compositions
and,withoutthebenefitof recentdating,on
ment (see discussionby Jamesand Henry [this issue]).Con- the assumption
that extensionand magmatismin Texaswere
temporaneously,magmatism with a clear arc trace element contemporaneous.
Thusinterpretation
of olderigneoussettings
signature, the southern Cordilleran basaltic andesite suite mustrecognizethatchangescanoccurrapidly.
(SCORBA) of Cameron et al. [ 1989], continuedto the west
Acknowledgments.Research support was provided by the U.S.
northernMexico. Similarto 28 Ma activityin Texas,SCORBA
Bureauof Mines grantG1194148 to the Texas Mining and Mineral
appearsto be partof a bimodalsuitethatincludesminorevolved Resources
ResearchInstituteandby the COGEOMAP programof the
rhyolites.The overall pattern and characteristics
of 28 Ma
magmatismin the southernCordillerasuggestthat regionally,
the arc did not extinguishabruptly.Indeed,subductioncontinuedalongmuchof thecontinentalmarginto the west,ceasing
only whenthe southern
triplejunctionjumpedto nearthemouth
of the presentGulf of Californiaabout12.5Ma [Mammerickx
and Klitgord, 1982]. Nevertheless,the contemporaneity
of the
stressandcompositional
changessuggestthatthey arerelated,
possiblyto the encroachment
andcollisionof the EastPacific
Riseandpaleotrench
about29 Ma [StockandMolnar, 1988].
By 24 Ma, igneousactivity in Texas consistedexclusively
of alkalicbasalts,typicalof continentalrifts, that accompanied
major Basin and Range faulting. The basaltsare relatively
primitive, as shownby high Mg numbersandNi contents,and
have Nb and Ta highs on spidergrams.
These basaltshave
U.S. GeologicalSurvey (CooperativeAgreement14-08-0001-A0408
and 14-08-0001-A0662).We thankC. E. ChapinandA. W. Laughlin
for reviews.
Aguirre-Diaz,G., and F. W. McDowell, Natureandtimingof faulting
in the southernBasin andRange,central-eastern
Durango,Mexico,
Eos Trans AGU, 69, 1412-1413, 1988.
Albritton, C. C., and J. F. Smith, Geologyof the SierraBlancaarea,
HudspethCounty,Texas,U.S. Geol.Surv.Prof. Pap., 479, 131 pp.,
1965.
Aldrich, M. J., Jr., C. E. Chapin,and A. W. Laughlin,Stresshistory
and tectonicdevelopmentof the Rio Grande rift, New Mexico,
J. Geophys.Res., 91, 6199-6211, 1986.
Anderson,J. L., Proterozoicanorogenicgraniteplutonismof North
America,Mere.Geol.•oc. Am.,161, 133-154,1983.
Hm,m¾rr At.: Mm42•ozoxc StraussEvoumo• ANUMnO•'nSM
13,559
Gans,B., G. A. Mahood,andE. Schermer,Synextensional
magmafism
in the Basin and Rangeprovince;a easestudyfrom the eastern
GreatBasin,Spec.Pap. Geol.Soc.Am.,233, 53 pp., 1989.
Am. Abstr.Programs,6, 484, 1974.
Angelier,J., Determination
of the meanprincipalstressfor a given Gastil, R. G., G. J. Morgan, and D. Krummenacher,The tectonic
history of peninsular California and adjacent Mexico,. in The
faultpopulation,
Tectonophysics,
56, 17-26, 1979.
GeotectonicDevelopmentof California, edited by W. G. Ernst,
Atwater,T., Implicationsof plate tectonicsfor the Cenozoictectonic
evolutionof westernNorth America, Geol. Soc.Am. Bull., 81, 3513pp. 285-306, Prentice-Hall,EnglewoodCliffs, N.J., 1981.
Glazner, A. F., and J. M. Bartley, Timing and tectonicsetting of
3536, 1970.
Tertiary low-angle normal faulting and associatedmagmafismin
Barker,D. S., NorthernTrans-Pecos
magmaticprovince:Introduction
the southwesternUnited States,Tectonics,3, 385-396, 1984.
andcomparison
withtheKenyaRift, Geol.Soc.Am.Bull.,88, 14211427, 1977.
Grant,G. J., andJ. Ruiz, The Pb-Zn-Cu-Agdepositsof the Crranadena
Mine, San Francisco del Oro-Santa Barbara district, Chihuahua,
Berge,T., Structuralevolutionof thenortheastern
Chihuahuatectonic
Mexico, Econ. Geol., 83, 1683-1702, 1988.
belt, in GeologicalStudiesof the CordilleranThrustBelt, vol. 1,
editedby R. B. Powers,pp. 451-457, RockyMountainAssociation Gregory,J. L., Volcanic stratigraphyand K-At agesof the Manuel
Benavides area, northeastern Chihuahua, Mexico, and correlations
of Geologists,
Denver,Colo., 1982.
with the Trans-PecosTexasvolcanicprovince,M. S. thesis,78 pp.,
Best, M. G., Early Miocene changein dLmagfion
of least principal
Um'v. of Tex. at A,•fin, !981.
stress,southwestern
L¾.,.'ted
States:Conflictingivfferences
,,C?om
•,•kes
Gross,W. H., New ore discoveryand sourceof silver-goldveins,
andmetamorphic
core-detachment
fault terranes,Tectonics,7, 249Guanajuato,Mexico, Econ. Geol., 70, 1175-1189, 1975.
259, 1988.
Cameron,K. L., G. J. Nimz, D. Kuenz, S. Nierneyer,and S. Gunn, Hagstrom,J. T., and D. A. Sawyer,Late CretaceouspalcomagnetiSm
Anderson,T. H., andL. T. Silver,Late Cretaceous
plutonismin Sonora,
Mexicoanditsrelationship
tocircum-Pacific
magma•m,Geol.Soc.
Southern Cordilleran basaltic andesitc suite, southern Chihuahua,
Mexico: A link betweenTertiary continentalarc and flood basalt
magmatismin North America,J. Geophys.Res.,94,. 7817-7840,
1989.
Cepeda,
J.C.,andC.'D. Henry,
Oligoeene
volcanism
andm•fiple
calderafortrationin the ChinaftMountains,PresidioCounty,Texas,
Rep.lnvest.135, 32 pp., Bur. of Econ.Geol.,Univ. of Tex. at Austin,
1983.
Chamberlin,R. M., Cenozoic domino-stylecrustalextensionin the
LemitarMountains,New Mexico:A sunmm•, Field Conf.Guide&,
and clockwise rotation of the Silver Bell Mountains, south central
Arizona,J. Geophys.Res.,94, 17,847-17,860, 1989.
Halliday, A. N., M. Arialion, I. Parsons,A. P. Dickin, and M. R. W.
Johnson,Syn-orogenicalkaline magmatismand its relationshipto
the Moine ThrustZone and the theruralstateof the Lithospherein
NW Scotland, J. Geol. Soc. London, 144, 611-617, 1987.
Hancock, L., and T. G. Bevan, Brittle modes of foreland extension,
in ContinentalExtensionalTectonics,editedby M.P. Coward,J. F.
Dewey,andL. Hancock,Geol.Soc.Spec.Publ.London,28, pp. 127137, 1987.
Harlan, S. B., J. W. Geissman,J. G. Price,C. D. Henry, T. C. Onstott,
and J. Oldow, Palcomagneticdocumentationof the cessationof
Chapin,C. E., and S. M. Cather,Eocenetectonicsandsedimentation
Chihuahuan fold thrust belt deformation, SW Texas, Geol. Soc. Am.
in the ColoradoPlateau-RockyMountain•a, in RockyMountain
Abstr.Programs,19, 810, 1987.
Foreland Basinsand Uplifts, editedby J. D. Lowell, pp. 33-56,
Heldrick,T. L., andS. R. Titley, Fractureanddikepatternsin •de
RockyMountainAssociation
of Geologists,
Denver,Colo., 1983.
plutonsand their structuraland tectonicimplications,in Advances
Chuchla,R. J., Reconnaissance
geology of the Sierra Rica area,
in Geology of Porphyry Copper Deposits, SouthwesternNorth
Chihuahua,Mexico, M.S. thesis,199 pp., Univ. of Tex. at Austin,
1981.
America,editedby S. R. Titley, pp. 73-91, Univ. of Ariz., Tucson,
1982.
Coney,P. J., Cordillerantectonicsand North Americanplate motion,
Am. J. Sci., 272, 603-628, 1972.
Henry, C. D., Geologyand geochronology
of the granitebatholithic
complex,Sinaloa,Mexico, Ph.D. dissertation,144 pp., Univ. of
Coney,P. J., and S. J. Reynolds,CordilleranBenioff zones,Nature,
N.M. Geol. Soc., 34, 111-118, 1983.
Tex. at Austin, 1975.
270, 403-406, 1977.
Damon, E., M. Shafiqullah,and K. F. Clark, Age trendsof igneous
activity in relation to metallogenesis
in the southernCordillera,
Ariz. Geol.Soc.Dig., 14, 137-154, 1981.
Dasch,E. J., R. L. Armstrong,andS. E. Claimugh,Age of Rim Rock
dike swarm, Trans-Pecos Texas, Geol. Soc. Am. Bull., 80, 18191824, 1969.
Henry, C. D., Late CenozoicBasin and Range structurein western
Mexico adjacentto the Gulf of California,Geol.Soc.Am. Bull., 101,
1147-1156, 1989.
Henry,C. D., andG. Fredrikson,Geologyof a partof southern
Sinaloa,
Mexicoadjacent
totheGulfof California,
Geol.Soc.Am.MapChart
Ser., MCH062, 1987.
Delaney,T., D..D. Pollard,J. I. Ziony, andE. H. McKee, Field rela- Henry,C. D., andF. W. McDowell, Geochronology
of magmatismin
tionsbetweendikesandjoints:Emplacementprocesses
andpalcotheTertiaryvolcanicfield, Trans-Pecos
Texas,Guide&23, pp. 99122, Bur. of Econ. GeoL, Univ. of Tex. at Austin, 1986.
stressanalysis,J. Geophys.Res.,91, 4920-4938, 1986.
Dickinson,W., M. A. Klute, M. J. Hayes,S. U. Janecke,E. R. Lundin, Henry, C. D., and J. G. Price, Variationsin calderadevelolm•nt in
the Tertiary volcanicfield of Trans-PecosTexas,J. Geophys.Res.,
M. A. McKittrick, and M.D. Olivares, Palcogeographicand
89, 8765--8786, 1984.
palcotectonic
settingof Lararnidesedimentary
basinsin the central
RockyMountainregion,GeoLSoc.Am.Bull.,100, 1023-1039,1988. Henry,C. D., andJ. G. Price,Early BasinandRangedevelopment
in
Drewes, H., The Cordilleran orogenic belt between Nevada and
Trans-PecosTexas and adjacent Chihuahua:Magmatism and
Chihuahua,Geol. Soc.Am. Bull., 89, 641-657, 1978.
orientation,timing, and style of extension,J. Geophys.Res., 89,
6213-6224, 1986.
Drier, J., Regionaltectoniccontrolsof epithenmlveinsin thewestern
U.S. andMexico,Ariz. Geol.Soc.Dig., 15, 28-52, 1984.
Henry, C. D., and J. G. Price, The ChristmasMountains caldera
Dunham,K. C., The geologyof theOrganMountains,Bull.N.M. Bur.
complex,Tmns-P•os Texas: The geologyand developmentof a
MinesMiner. Resour.,11,272 pp., 1935.
laccocaldera,Bull. Volcanol., 52, 97-112, 1989.
Elston,W. E., Subducfion
of youngoceaniclithosphere
andextensional Henry, C. D., F. W. McDowell, J. G. Price, and R. C. Smyth,
orogenyin southwestern
North Americaduringmid-Tertiarytime,
Compilationof poressiren-argon
agesof Tertiary igneousrocks,
Tectonics,3, 229-250, 1984.
Trans-Pecos
Texas,Geol. Circ. 86-2, 34 pp., Bur. of Econ.Geol.,
Univ. of Tex. at Austin, 1986.
Ememmn,S. H., andR. Marrett, Why dikes?,Geology,18, 231-233,
1990.
Henry,C. D., $. G. Price,andD. Miser,GeologyandTertiaryigneous
Engebretson,
D.C., A. Cox, and G. A. Thompson,Correlationof
activity of the Hen Egg Mountain and Christmas Mountains
plate motionswith continentaltectonics:Laramideto Basin-Range,
quadrangles,
Trans-Pecos
Texas:Rep.Invest.183, 105pp., Bur. of
Tectonics,3, 115-120, 1984.
'
Erdlac, R. J., A Laramide-agepush-upblock: The structuresand
fomtion of the Terlingua-Solitariostmctmal block, Big Bend
region,Texas,Geol. Soc.Am. Bull., 102, 1065-1076, 1990.
Feraud,G., G. Giannerini,andR. Campredon,Dyke swarmsaspalcostressindicatorsin areas adjacentto continentalcollision zones:
ExamplesfromtheEuropeanandnorthwest
Arabianplates,in Mafic
dykeswarms,Geol.Assoc.Can. Spec.Pap., 34, 273-278, 1987.
Econ. Geol., Univ. of Tex. at Austin, 1989.
Henry,C. D., J. G. Price,J. N. Rubin,and S. E. Laubach,Casestudy
of an extensivesiliciclava:The BracksRhyolite,Trans-Pecos
Texas,
J. Volcanol. Geotherm. Res., 43, 113-132, 1990.
Hoover,J. D., S. E. Ensenat,C. G. Barnes,andR. Dyer, Early TransPecosmagmatism:Petrologyand geochemistry
of •ne
intrusive
rocksin the El Pasoarea,Field Conf. Guide&, N.M. Geol. Soc.,39,
109-118, 1988.
13,560
Hmm¾ LrrAL.: MID-CENozoIc STR•S EvotJynoNANDMAO!•.ATISM
Ingersoll,R. V., Triple-junction
instabilityascausefor late Cenozoic Price, J. G., and C. D. Henry, Dikes in Big Bend NationalPark:
extensionandfragn•ntafionof theWesternUnitedStates,Geology,
Petrologic
andtectonic
significance,
centennial
fieldguide,pp.43510, 621-624, 1982.
440, Geol. Soc.Am., Boulder,Colo., 1988.
James,E. W., and C. D. Henry, Compositional
changesin Trans- Price,J. G., C. D. Henry,A. R. Standen,
J. S. Posey,Originof silverPecosTexasmagmascoincidentwith Cenozoicstressrealignment,
copper-lead
depositsin red-bedsequences
of Trans-Pecos
Texas:
J. Geophys.Res., thisissue.
Tertiary minemlizationin Precambrian,Permian, and Cretaceous
Keith, S. B., Paleosubduction
geometriesinferredfrom Cretaceous
sandstones,
Rept. Invest. 145, Bur. of Econ. Geol., Univ. of Tex. at
and Tertiary magmaticpatternsin southwestern
North America,
Austin, 1985.
Geology,6, 516-521, 1978.
Price,J. G., C. D. Henry,D. S. Barker,andJ. N. Rubin,Petrology
Kunk, M. J., and C. D. Henry, A detailed chronologyof silicic
of theMarbleCanyonstock,Culberson
County,Texas,Guideb.23,
volcanism in the Davis Mountains, Trans-PecosTexas, Geol. Soc.
Am. Abstr.Programs,22, A351, 1990.
Laughlin,A. W., C. Kress,and M. J. Aldrich, K-At agesof dike
rocks, Big Bend National Park, Texas,Isochron West,35, 17-18,
1982.
pp. 303-319, Bur. of Econ. Geol., Univ. of Tex. at Austin, 1986.
Price, J. G., C. D. Henry, D. S. Barker,and D. F. Parker,Alkalic
rocksof contrasting
tectonicsettings
in Trans-Pecos
Texas,Spec.
Pap. Geol. Soc.Amer., 215, 335-346, 1987.
Price,J. G., S. T. Conlon,and C. D. Henry, Tectoniccontrolson
Laughlin, A. W., M. J. Aldrich, and D. T. Vaniman, Tectonic
orientation and size of epithetreal veins, in North American
implicationsof mid-Tertiarydikes in west-centralNew Mexico,
Conference
on TectonicControlsof Ore Depositsandthe Vertical
Geology,11, 45-48, 1983.
andHorizontal
ExtentofOreSystems,
Proceedings
Volume,
pp.36Lindgren'W., Theigneousgeologyof theCordilleras
anditsproblems,
46, editedby G. Kisvarsanyiand S. K. Grant,Univ. of Missouri,
Rolla, 1988.
in Problemsof AmericanGeology,pp. 234-286, Yale University
Press,New Haven, Conn., 1915.
Reagan,
M. K., andJ.B. CAH,
Coexisting
calcalkaline
andhigh-niobium
Lipman,P. W., Volcano-tectonic
settingof Tertiaryore deposits,
basalts
fromTurfialba
Volcano,
CostaRica:Implications
forresidual
southern
RockyMountains,
Ariz. Geol.Soc.Dig., 14, 199-214,1981.
timnates
in arc magmasources,
J. Geophys.
Res.,94, 46194633,
Lipman, P. W., The Miocene Questacaldera,northernNew Mexico:
1989.
Relationto batholitheraplacement
and associated
molybdenum Rehrig,W. A., andT. L. Heldrick,
Regional
tectonic
stress
duringthe
mineralization, in Genesis of Rocky Mountain Ore Deposits,
Laramideandlate Tertiaryintrusiveperiods,BasinandRange
Changes
WithTimeandTectonics,
Proceedings
oftheDenverRegion
province,Arizona,Ariz. Geol.Soc.Dig., 10, 205-228, 1976.
ExplorationGeologistsSocietySymposium,
pp. 133-148, Denver Seaget,W. R., Geologyof OrganMountainsandsouthern
SanAndres
RegionExploration
Geologists
Society,Denver,Colo.,1983.
Mountains,
NewMexico,Mere.N.M. Bur.MinesMiner.Resour.,
36,
Lipman, P. W., H. J. Prostka,and R. L. Christiansen,Evolving
97 pp., 1981.
subduction
zonesin the westernUnitedStates,asinterpreted
from Severinghaus,
J., and T. Atwater,Cenozoicgeometryand thermal
igneousrocks,Science,148, 821-825, 1971.
Lipman, P. W., F. S. Fisher, H. H. Mehnert, C. W. Naeser,and T. A.
Steven,Multiple ages of mid-Tertiarymin,mlizationand alteration in the westernSan Juanmountains,Colorado,Econ. Geol., 71,
571-588, 1976.
Loring,A. K., andR. B. Lodng,K/At agesof middleTertiaryigneous
rocksfrom southernNew Mexico, IsochronWest,28, 17-19, 1980.
Mamrnerickx,J., and IC D. Klitgord, NorthernEast Pacific Rise:
Evolutionfrom 25 m.y.B.P. to the present,J. Geophys.
Res.,87,
6751-6759, 1982.
Maxwell, R. A., J. T. Lonsdale,R. T. Hazzard, and J. A. Wilson,
Geologyof Big BendNationalPark,Brewster
County,Texas,Publ.
stateof subductingslabsbeneathwesternNorth America,in Basin
andRangeextensional
tectonics
nearthelatitudeof Las Vegas,
Nevada,editedby B. P. Wernicke,Mem. Geol.Soc.Am., 176, 1990.
Sewell,C. R., TheCandela
andMonclova
beltsof igneous
intmsions-a petrographic
provincein NuevoLeonandCoahuila,Mexico,Spec.
Pap. Geol. Soc.Am., 121,273, 1968.
Smith,D. M., T. Albinson,andF. J. Sawkins,Geologicandfluid
inclusion
studies
of theTayoltita
silver-gold
veindeposit,
Durango,
Mexico, Econ.Geol., 77, 1120-1145, 1982.
Stewart,J. A., Basin-rangestructurein westernNorth America, a
review,in CenozoicTectonicsandRegionalGeophysics
of the
WesternCordillera,editedby R. B. Smith,andG. P. Eaton,Mem.
6711, 320 pp., Univ. of Tex. at Austin,1967.
Geol. Soc.Am., 152, 1-13, 1978.
McGarr,A.M., Analysisof stressbetweenprovinces
of constant
stress, Stock,J., andP. Molnar,Uncertainties
andimplications
of theLate
J. Geophys.Res., 87, 9279-9288, 1982.
Cretaceous
andTertiarypositionsof NorthAmericarelativeto the
Mcintosh,W. C., Timinganddistribution
of ignimbritevolcanism
in
Farallon,Kula,andPacificplates,Tectonics,
7, 1339-1384,1988.
theEocene-Miocene
Mogollon-Datilvolcanicfield,Mem.N.M. Bur. Wernicke,B. P., R. L. Christiansen,
P. C. England,andL. J. Sonder,
Mines Miner. Resour., 46, 58-59, 1989.
Tectonomagmatic
evolutionof Cenozoicextensionin the North
McKenzie,D. P., Speculations
on the consequences
and causesof
AmericanCordillera,in ContinentalExtensionalTectonics,edited
plate motions,Geophys.J. R. Astron.Soc.,18, 1-32, 1969.
by M.P. Coward,J. F. Dewey,andL. Hancock,Geol.Soc.Spec.
Moustafa,A., Analysisof Laxamideand youngerdeformation
of a
Publ. London, 28, 203-221, 1987.
segmentof theBig Bendregion,Texas,Quad.Map 54, Bur.of •.
Whalen,J. B., K. L. Cuttle,andB. W. Chappell,A-typegranites:
Geol., Univ. of Tex. at Austin, 1988.
Geochemicalcharacteristics,discriminationand petrogenesis,
Muehlberger,
W. R., TexasLineanentrevisited,Field Conf.Guideb.,
Contrib. Mineral Petrol., 95, 407-419, 1987.
N.M. Geol. Soc.,31, 113-121, 1980.
Wilson, J. A., Vertebratebiostratigraphyof Trans-PecosTexas,
Muller, O. H., and D. D. Pollard,The stressstatenearSpanishPeaks,
Publ. 71-59, WestTex.Geol.Soc.,pp. 157-166, 1971.
Coloradodetermined
froma dikepattern,PureAppLGeophys.,
115, Wust,S. L., Regionalcorrelationof extensiondirectionsin Cordilleran
69-86, 1977.
metamorphic
corecomplexes,
Geology,14, 828-830, 1986.
Nakamura, K., Volcanoesas possibleindicatorsof tectonicstress Yates,R. G., andG. A. Thompson,
Geologyandquicksilverdeposits
orientation---principle
andproposal,J. Volcanol.Geotherm.Res.,2,
of the Terlinguadistrict,Texas,U.S. Geol. Surv.Prof. Pap., 312,
1-16, 1977.
114pp., 1959.
Nakamura,K., and S. Uyeda, Stressgradientin arc-backarc regions Zoback,M., R. E. Anderson,
andG. A. Thompson,
Cainozoic
evolution
andplatesubduction,
J. Geophys.Res.,85, 6419-6428, 1980.
of thestateof stressandstyleof tectonism
of theBasinandRange
Nelson,M. R., andC. H. Jones,Paleornagnetism
andcrustalrotations
province of the western United States, Philos. Trans. R. Soc.
alonga shearzone,LasVegasRange,southern
Nevada,Tectonics,
6,
London,Ser. A, 300, 407-434, 1981.
13-33, 1987.
Zoback,M. L., et al., Globalpatternsof tectonicstress,Nature,341,
Newcomer,R., T. H. Criordano,and W. R. Seager,Hydrothermal
291-298, 1989.
alterationandmineralization
of theSugarloaf
Peakquartzmonzonite,
Dona Ana Co., N.M., Geol. Soc.Am. Abstr.Programs,15, 276,
C. D. HenryandE. W. James,
Bureau
of Economic
Geology,
University
1983.
of Texasat Austin, Austin, TX 78713.
Pollard,D. D., andA. Aydin, Progress
in understanding
jointingover
J. G. Price,NevadaBureauof Mines and Geology,Universityof
thepastcentury,Geol. Soc.Am. Bull., 100, 1181-1204, 1988.
Nevada,Reno, NV 89557.
Price, J. G., and C. D. Henry, StressorientationsduringOligocene
volcanism in Trans-PecosTexas: Timing the transitionfrom
(ReceivedJuly 2, 1990;,
Laramidecompression
to Basinand Rangetension,Geology,12,
revisedOctober25, 1990;
238-241, 1984.
acceptedJanuary10, 1991.)